This invention relates in general to punching or perforating apparatus and is more particularly directed to apparatus for perforating plastic film. The invention is directed also to a resiliently flexible self-sharpening punch head for such apparatus.
As is known to those skilled in the art, flexible plastic film or plastic bags formed from materials such as polyethylene, are frequently used in groceries and supermarkets to package bulk food items such as fruit, vegetables, grain, candies, etc. These bags are typically rolled on a spindle, and the user unrolls one bag as needed. The plastic film bag is torn at a row of perforation that separates each bag from the next, and a quantity of bulk food or an article such as a loaf of bread, is placed in the bag.
U.S. Pat. Nos. 3,654,829; 4,160,396 and 4,308,774 each disclose apparatus for perforating thermoplastic material utilizing a serrated knife edge to cut or punch holes in a plastic film. Such cutters have been formed from metal and require frequent periodic sharpening so that they will be keen enough to cut through the plastic. During the cutting operation, the plastic material is supported upon a backing plate having an opening formed therein which is larger than the cutting tool. The backing plate so formed enables the cutting tool to move through the plastic material while the material is held in position against the plate.
The metal cutting tools as used in the prior art are satisfactory only so long as the cutting tool or head remains sharp, and the tool and opening in the backing plate are maintained substantially in registry out of contact during the high speed punching operation. The hole in the backing plate must be relatively close to the same size as the punching die or cutting tool to prevent stretching of the plastic material. If proper alignment is not maintained, the teeth of the cutter will be broken, requiring replacement of the entire cutting head. In addition, the cutting of the thermoplastic material rapidly wears the metal cutting surfaces thus dulling the cutting tool and requiring frequent regrinding or resharpening of the cutting edge. The wear soon becomes excessive and the tool must be replaced.
A circular cutting head of a plastic resin material has been proposed by the inventor hereof, and this is described in his U.S. Pat. No. 4,653,372 of Mar. 31, 1987. That cutting head is formed of a semirigid material such as Delrin (acetal homopolymer). The head there is intended for punching circular holes in plastic material. That cutting head is sufficiently flexible so that the teeth deflect and bend, rather than break off, when the teeth strike the backing plate. The teeth are also self-sharpening as the head wears, so regrinding is unnecessary over the life of the cutting head.